Evacuated sample collection tubes have been in general usage in the United States for almost fifty years. Initially, as disclosed in U.S. Pat. No. 2,460,641 to Klienert, the sample collection tube was simply an evacuated glass test tube with a resilient stopper and intended for use in blood collection. As practitioners recognized the utility of these evacuated blood collection tubes, (trade named "VACUTAINER" and available from Becton, Dickinson and Company, Franklin Lakes, N.J.) the tubes are now supplied with various additives already in them to prepare the blood specimen for a particular test. The tubes now are also evacuated to selected less than atmospheric pressures to provide a preselected volume of blood drawn. The most widely used tubes are evacuated to provide a blood withdrawal volume of about four and one half milliliters.
One commonly used additive tube is termed a "coagulation" tube. According to a widely followed convention, these coagulation tubes contain a sufficient amount of buffered citrate in aqueous saline to provide a ratio of about 9:1 of blood to aqueous citrate. Coagulation tubes are often used to collect blood samples to assess the need for or to adjust the dosage of anticoagulant medications. When the freshly drawn blood contacts the citrate, the cellular fraction, i.e., red blood cells, platelets and white blood cells precipitate, leaving plasma as the supernatant liquid after the tube is centrifuged. Commonly, an aliquot of this plasma is then subjected to treatment with a coagulation agent such as thromboplastin reagent or similar and the time to clot is measured. These time to clot determinations are called the prothrombin time ("P.T.") and activated partial thromboplastin time ("A.P.T.T.").
More recently, clinical testing practitioners have recognized the hazards associated with blood-borne pathogens and manufacturers have begun to supply blood collection tubes from materials less likely to be broken by inadvertent mishandling than the original glass tubes. Evacuated blood collection tubes are now supplied formed from thermoplastic resins such as polyethylene terephthalate (PET) and similar materials. While these thermoplastic resin tubes have reduced the chance of breakage by inadvertent mishandling, the use of thermoplastic resin tubes to replace glass causes other problems. Some thermoplastic materials were found not to be capable of withstanding the stress of vacuum, other materials are permeable in varying degrees to gases such as oxygen, water vapor and the like. When the tube material is permeable to gases, the transmission into the tube of oxygen or nitrogen results in gradual loss of the vacuum. In the case of the citrate coagulation tubes and other tubes with aqueous additives, loss of water vapor through the tube wall during shelf storage results in changing the concentration of the aqueous additive. In the case of the coagulation tube, once the concentration of the aqueous saline buffer changes appreciably, the ratio of blood drawn to the aqueous saline buffer no long follows the convention of the 9:1 ratio, rendering the tube unsatisfactory for use. The loss of water through the tube wall has proved to be the limiting factor for thermoplastic coagulation tube shelf life.
The water loss problem through thermoplastic coagulation tubes has been addressed by two current commercial suppliers. One supplier, Terumo, Elkton, Md. supplies their thermoplastic coagulation tubes in a sealed tub containing about 15 tubes. As long as the tub is sealed, the water loss through the tubes is controlled, providing a usable shelf life. Once the tub is unsealed, the tubes begin to lose water through the tube walls. As a result, practitioners need to use up these tubes within a few weeks once the tub is opened. Another supplier, Greiner GMBH, Frickenhausen, Germany, provides a thermoplastic coagulation tube formed from two separate independently formed layers, one inside the other. In the Greiner tube, the inner tube is formed from polypropylene, a material that is substantially non-permeable to water vapor, and the outer layer is PET. The two layer tube provides a satisfactory shelf life by limiting water vapor transmission, but the manufacture and assembly of the two components into the two-layer tube is inefficient compared to a single layer tube.
If a single layer thermoplastic tube was available that limited water loss from tube additives, thus providing a shelf-life similar to that of borosilicate glass tubes with similar additives, the art of blood collection tubes would be advanced. Such a tube is disclosed herein below.